Adds a cubic Bezier spline to the path from the current point to position x3
,y3 in user-space coordinates, using x1,
y1 and x2, y2 as the control
points. After this call the current point will be x3, y3.

If there is no current point before the call to CairoContext::curveTo()
this function will behave as if preceded by a call to CairoContext::moveTo()
(x1, y1).